I'm in the process of installing a set of ported heads on my stock 82,000 mile LT4 shortblock. I figured I would calculate the compression ratio. I used a micrometer and bridge to find out the average piston deck height...well everything looked hunky dory until #7... .054? That is about .020 off what it should be. The engine ran fine when disassembled, but it has been run hard. I checked and rechecked the measurement, trying different locations, ensuring no dirt particles were causing an error, etc. Even visually, the piston looks further down in the hole than the other 7.

What could cause this...assume the piston and rod are within spec for length? Only thing I could come up with is a failing rod bearing...

Anybody have any thoughts on this? I did not plan to touch the shortblock other than a new oil pan... If it is .020 in the hole from rod clearance then that would mean it is has a rod bearing clearance of ~.050! Does that makes sense? Again oil pressure was fine, no misfires...only question I have is I had recently dragged the car and it was down about 4 mph from the begginning of the year...I was chocking it up to different weather conditions and worn valvesprings...

1 0.024 2 0.029
3 0.027 4 0.027
5 0.03 6 0.027
7 0.054 8 0.026

Thanks

:(

 

Unless the engine exhibited a loud knocking sound before you began the work I doubt if the rod bearing is a problem.

The piston crown/deck clearance is a function of crank throw radius, rod length, and "compression height" - the distance between the center of the pin and the piston crown, so there must be a tolerance issue with one of those components, or a tolerance stack up issue. Given your measurement, something must be way out of blueprint tolerance range.

Duke

 

I agree with what you are saying.

The engine definitely didn't have any unusual noises or operation issues. I simply pulled the engine to add the oil pan and the heads.

I actually had wanted to do a compression reading (because I have one) before tearing it apart, but it didn't happen.

I would like to keep the shortblock running for one more year then a 383 will be installed (already have a block)...I wonder if it will last...

:(

 

Out of tolerance dimensions that result in excess deck clearance shouldn't lead to a failure, assuming that's the problem.

It just means the CR on that cylinder is about a quarter to half point lower than the others, so it's down a couple of horsepower from the other seven. I doubt if you could even detect it on a cylinder balance test.

Duke

 

A bent rod is a possiblity. When I took my original engine out of my car, and disassembled it, I had the same situation. Turned out to be a bent rod, from using starter fluid(ether) to help start it a couple of times on real cold days is my theory. The engine an fine like that though before I took it out.

 

It is possible to pull just a single piston and rod out of one cylinder. You have to be able to take the pan off, and then unbolt the rod bolts and cover the ends with rubber vacuum plugs or something, and then carefully pound it out the top with a wooden stick and rubber mallet. Check with an engine builder though to see if you even need to worry about it. I'd at least want to pull the pan and visually examine it for evidence of bending, but I'm not sure you'd be able to see that little of an amount.

There is one other thing that can cause that, and that's a bent wrist pin in the piston. Once again, the piston and rod have to come out to fix that. Anyway, get advice from a good engine builder to make sure. There may be a tolerance range and you may be within.